In high speed communications systems, data is transmitted across transmission lines. These transmission lines run between transmitters and receivers, which are typically semiconductor chip devices. To minimize distortion of the transmitted signal, transmission lines require proper transmission line termination.
Transmission line termination is generally achieved through a fixed, off-chip resistance that precisely matches the transmission line characteristic impedance. Such transmission line termination is off-chip because of difficulties in creating precision on-chip resistances. Typically, tolerances of less than five percent are required for transmission line termination. These kind of tolerances are difficult to achieve with high volume semiconductor processing methods, for example, because of variations in sheet resistance and line width control.
Furthermore, many communications applications use high density digital CMOS technology for high speed. As speed performance improves, process elements such as poly and moat resistors are often unavailable to circuit designers because of the use of silicide to minimize sheet resistances. Thus, to create resistors, additional mask levels, or the use of analog specific processes, are required.
The use of off-chip precision resistors for transmission line termination presents significant drawbacks. For example, off-chip resistors require significant circuit board space, and increase circuit board layout complexity and cost. Furthermore, the distances between off-chip resistors and signal generators or receivers are fairly large, although these distances should be as small as possible to reduce stub length reflection problems.